U.S. Pat. No. 3,893,869 discloses a cleaning system wherein very high frequency energy is employed to agitate a cleaning solution to loosen particles on the surfaces of semiconductor wafers. Maximum cleanliness is desired in order to improve the yield of acceptable semiconductor chips made from such wafers. This cleaning system has become known as megasonic cleaning, in contrast to ultrasonic cleaning, in view of the high frequency energy employed. Ultrasonic cleaners typically generate random 20-40 kHz sonic waves that create tiny cavities in a cleaning solution. When these cavities implode, tremendous pressures are produced which can damage fragile substrates, especially wafers. Megasonic cleaning systems typically operate at a frequency over 20 times higher than ultrasonics, and consequently they safely and effectively remove particles from materials without the side effects associated with ultrasonic cleaning.
A number of improvements have been made to this system as initially outlined in the above-referenced patent, and several companies are now marketing such cleaning apparatus. One of these is Verteq, Inc. of Anaheim, California, the assignee of the invention disclosed and claimed in this document. One of the major improvements that helped make the product a commercial reality concerns the design of the transducer array which converts electrical energy into sound waves for agitating the cleaning liquid. The transducer is perhaps the most critical component of the megasonic cleaning system. The transducer array which has been developed over a number of years and has been marketed by Verteq for a number of years is mounted on the bottom of the process tank close to the components to be cleaned so as to provide powerful particle removal capability. The transducer array includes a strong, rigid frame suitable for its environment, and in one form includes a very thin layer of tantalum, which is a ductile, acid-resisting metallic element, spread over the upper surface of the frame.
A pair of spaced rectangular ceramic transducers are positioned within a space in the plastic frame and bonded by electrically conductive epoxy to the lower side of the tantalum layer extending over the space in the frame. The transducer has a coating of silver on its upper and lower faces that form electrodes. RF (radio frequency) energy approximately 800 kHz is applied to the transducer by connecting one lead to the lower face of the transducer and by connecting the other lead to the layer of tantalum which is electrically conductive and which is in electrical contact with the upper silver coating of the transducer.
While megasonic cleaning systems employing this transducer array have enjoyed commercial success, improvements have been made recently wherein materials more durable than tantalum have been used for transmitting the megasonic energy. Such improvements are set forth in the above referenced U.S. patent application. In a preferred form of that invention, the transmitting material is in the form of a quartz or sapphire plate to which the transducers are bonded by a suitable epoxy which need not be electrically conductive.
In using megasonic cleaning apparatus of the types discussed above, a cassette of semiconductor wafers is typically immersed in a cleaning solution in a container, with the transducer array being mounted in the bottom wall of the container. The wafer carrier typically has an elongated rectangular opening in its bottom wall and it includes a structure forming a series of slots which engage the side lower edge portions of the wafers to support the wafers in spaced, substantially parallel relation, with the wafers being oriented substantially vertically. The megasonic energy is thus transmitted upwardly through the opening in the carrier to adjacent portions of both faces of the wafers to loosen contaminating particles on the surface of the wafers. To increase the exposure of the surfaces of the wafers to the megasonic energy, the carriers are moved transversely across the upwardly extending generally rectangular beam of megasonic energy.
While this approach is widely used, it has shortcomings. From a cleaning standpoint, it is difficult to adequately expose the edge portions of the wafers to the megasonic energy in view of the carrier structure that extends between the megasonic energy pattern and the edge portions of the wafers. Also, apparatus is needed for moving the carrier back and forth within the container, together with controls for controlling the rate and duration of the movement. Both the moving apparatus and the controls add considerably to the expense of the apparatus. Further, since the container must be sufficiently large to accommodate this movement of the carrier, container expense is significant, and more importantly, it is necessary to provide sufficient cleaning solution within the container, and the solutions needed are expensive.
Perhaps even a more important undesirable aspect of this arrangement is that the moving apparatus may generate particles of its own which can contaminate the wafers. Steps to minimize this possible source of contamination adds further to the expense of the apparatus. Also, it is in general desirable to minimize movement of wafers and thus minimize the risk of damage or breakage. Breakage, of course, further reduces the acceptable product yield obtained from the wafers, and adds to the cost of the acceptable products.
For all the foregoing reasons, a need exists for further improvements in megasonic cleaning apparatus. More specifically, it is desirable to: (1) do a better job of cleaning the wafers; (2) eliminate the need to move the wafers during the cleaning operation; (3) reduce the size of the cleaning container relative to the size of wafer carrier; (4) reduce the volume of cleaning solutions needed; and (5) thereby reduce the cost of the megasonic cleaning apparatus and the cost of the processed products.